A polymer electrolyte fuel cell (hereinafter abbreviated as “fuel cell”) uses as a polymer electrolyte a polymer membrane having ion conductivity.
When the fuel cell generates electric power, an oxidizing gas, such as air, flows through a gas passage of an electrically-conductive separator plate of a cathode of a unit cell, and a fuel gas, such as a hydrogen gas, flows through a gas passage of an electrically-conductive separator plate of an anode of the unit cell.
The unit cell is a minimum unit constituting a cell. The fuel cell includes: a member called an MEA constructed of a polymer electrolyte membrane, a fuel electrode and an air electrode; and electrically-conductive separator plates. A detailed configuration of such unit cell will be described later.
When the hydrogen gas flows through the gas passage, a part of the hydrogen gas diffuses in a diffusion layer in a gas diffusion electrode of the anode and reaches a catalyst reaction layer in the gas diffusion electrode. In the catalyst reaction layer, electrons are emitted from hydrogen molecules of the hydrogen gas, and the hydrogen gas is ionized and dissociated into hydrogen ions (H+; protons). The electrons emitted from the hydrogen molecules move through an external circuit to the cathode. The ionized and dissociated hydrogen ions penetrate through the polymer electrolyte membrane, and reach a catalyst reaction layer in a gas diffusion electrode of the cathode.
Meanwhile, when the oxygen gas in the air flows through the gas passage, a part of the oxygen gas diffuses in a diffusion layer in the gas diffusion electrode of the cathode and reaches the catalyst reaction layer in the gas diffusion electrode. In the catalyst reaction layer, oxygen molecules react with the electrons to become oxygen ions. Further, the oxygen ions react with the hydrogen ions to generate water.
To be specific, when the oxidizing gas (air; reactant gas) and the fuel gas (reactant gas) react with each other to generate water, the electrons are transferred from the fuel gas to the oxidizing gas, and an internal temperature of a fuel cell FC increases by reaction heat. Thus, the electrons are emitted outside from the anode, and flows in the external circuit as current. Moreover, by causing water or the like to flow in the electrically-conductive separator plate, the reaction heat is transferred outside by the water. As above, the fuel cell is a cogeneration device which causes the fuel gas, such as the hydrogen gas, and the oxidizing gas, such as air, to electrochemically react with each other to generate electricity and heat at the same time.
Next, a construction example of a conventional fuel cell stack formed by stacking a plurality of the above-described unit cells in series will be outlined.
A pair of electrically-conductive current collectors and a pair of insulating plates are disposed on both ends, respectively, of the fuel cell stack in order to collect the electricity generated by the fuel cell. Further, a pair of end plates are disposed on both ends, respectively, of the insulating plates so as to sandwich the fuel cell stack, the current collectors and the insulating plates. The end plates are fixed to each other by fastening bolts which penetrate through four corners of the insulating plates, the current collectors and the fuel cell stack, while applying a predetermined pressing force to the fuel cell stack. This secures appropriate seal performances of a reactant gas sealing gasket (will be described later) and a water sealing gasket (will be described later) in the fuel cell stack.
In the case of applying the pressing force to the fuel cell stack, it is important that variations in loads applied to the surface of an electrode portion (a region of the gas diffusion electrode) of the fuel cell stack are reduced as much as possible, i.e., the electrode portion of the fuel cell stack is pressed uniformly by the end plates.
Here, a prior art has been proposed, which intends to suppress the variations in loads applied to the surface of the fuel cell stack, in such a manner that: an X-type end plate is used such that elastic members (springs) are disposed between the end plate and the fuel cell stack; the arrangement of the fastening bolts and springs is devised; and the selections of spring constants are devised (see Patent Document 1).
Patent Document 1: Japanese Laid-Open Patent Application Publication 62-271364 (FIG. 2)